Autism spectrum disorder is currently defined as a single disorder characterized by impairments in social communication and the presence of restricted interests and repetitive behaviors. However, there is mounting evidence that autism represents a collection of overlapping neurodevelopmental disorders, resulting from a variety of neuroanatomical, neurophysiological, neuroimmunologic and/or genetic abnormalities. The PDN research program aims to characterize the behavioral and biological manifestations of these differing presentations of ASD and to identify their unique features in order to facilitate development of effective therapeutic and preventive interventions. During the reporting period, this project completed a longitudinal, phenotyping investigation of 106 young children (ages 1-6 years) with autism, 60 age and sex-matched typically developing controls and a group of 25 children with non-autistic developmental delays (Protocol 06-M-0102, NCT 00298246). Follow-up assessments were completed during this reporting period. Data from the baseline evaluations have explored the area of regression, finding that it appears to be a continuous variable, rather than the dichotomous categories previously described. Findings have also explored how autism symptoms relate to language development in this cohort, in which the majority was described as minimally verbal at baseline. Nonverbal cognitive ability was a strong predictor of language development over time, although social-communication symptoms do contribute to language development as well. In addition to the behavioral assessments, the study includes comprehensive medical and developmental histories; neuropsychological, medical and neurological evaluations; assays of blood, urine and cerebrospinal fluid (CSF) samples; and also a variety of specialized studies, including routine and overnight electroencephalograms (EEGs); modified polysomnography to evaluate sleep architecture; and magnetic resonance imaging (MRI scans); genetic assays; and dysmorphology evaluations. Since more than one-half of the children with autism were found to have abnormalities of sleep architecture, with particularly notable relative reductions in the percentage time spent in Rapid Eye Movement (REM) sleep, exploration of potential mechanisms and treatment relating to sleep abnormalities are continuing to be explored and are described in a separate project report (ZIAMH002914). We also analyzed data from MRI scans, which found signals of abnormal myelination in specific brain areas (i.e. corpus callousum) in children with autism, as well as other indicators of abnormalities in brain white matter and relative cortical thinning in specific key regions in the brain. Evidence of previously reported abnormalities of early brain growth was also investigated, both through imaging as well as head circumference records, with data indicating previous findings may have spuriously overestimated increased head size in autism due to inadequate norms). Longitudinal data are currently being explored, with emphasis on using measures of change in cortical thickness and surface area, in addition to cortical volume. As expected, genetic abnormalities were found in approximately 10% of the subjects and the aberrations are being evaluated for clinical significance. Further, we are engaged in a collaboration to conduct whole exome sequencing of the cohort, and are in the process of analyzing the relationship of sequencing data and phenotypic variations. More information about this study (Protocol 06-M-0102, NCT 00298246) may be found at http://clinicalstudies.info.nih.gov/. Other phenotyping studies are also underway or have been recently completed. Each of them shares the goal of providing a deeper understanding of the autism phenotype and its relationship to etiology, clinical course and outcome. One of these projects is a prospective, longitudinal investigation of toddlers considered to be at-risk for ASD because of the presence of early language delays (Protocol 11-M-0144, NCT01339767). This study aims to delineate early communicative impairments that predict ASD and to distinguish these from non-specific markers of (non-autistic) developmental delays. The investigation also examines the relationship of communication impairments to abnormalities of brain structure and function assessed by electroencephalograms (EEGs), structural MRIs, and functional MRI scans. The longitudinal assessments also include comprehensive behavioral assessments designed to profile strengths and weaknesses in communication and other domains. The goal of these assessments is to identify specific risk and resilience factors for ASD. More information about this study may be found at: http://www.clinicaltrials.gov/ct2/show/NCT01339767?term=toddlers+autism&rank=2. A related investigation uses near-infrared spectroscopy (NIRS) to perform functional neuroimaging in children who are too young or behaviorally-challenged to tolerate functional MRI scanning procedures -- a key group that is missing from much of the neuroimaging literature on autism. Using tasks of inhibition and including comparison groups of children with attention-deficit hyperactivity disorder and typically developing children, this study seeks to obtain reliable and informative brain activation data on children with autism as young as 4 years old. Findings from typically developing children indicate specific parameters of cerebral autoregulation may be promising to investigate in relation to developmental differences. A final area of phenotyping investigation involves behavioral assessments and the natural history of individuals with specific genetic disorders purported to be highly associated with autism, such as Phelan McDermid Syndrome, Smith-Lemli-Opitz Syndrome and others. These efforts include participation in a Rare Disease Clinical Research Network Developmental Synaptopathies Consortium. More information may be found at https://www.clinicaltrials.gov/ct2/show/NCT02461420?term=NCT02461420&rank=1 and https://www.rarediseasesnetwork.org/studies/index.htm#. In depth characterization, including overnight EEG/sleep studies and MRIs are being conducted. Additionally, through collaborations with NHGRI, subjects with serious rare known and unknown genetic disorders are evaluated to find common and unique patterns of developmental and behavioral strengths and weaknesses, as well as masked or explicit symptoms of ASD. The behavioral assessments are conducted as part of PDN's Natural History investigation (Protocol 13-M-0028, NCT01778504), which is described at: http://clinicalstudies.info.nih.gov/cgi/detail.cgi?A_2013-M-0028.html